I would actually not try to add C++ code if you do not have basic knowledge about programming and getting your code running within the context of the engine.
Also, you cannot just paste my code into your project. You would at least have to edit the Slate widget setup - which is unintuitive to say the least.
Some users suggested using “streaming levels” to get async loading but I have not tried that yet and I suspect other problems to show up that need to be solved then…
Marc